The present invention relates to an apparatus for determining oxygen concentration, which measures the amount of oxygen contained in the exhaust gas of, for example, an internal combustion engine, so as to control the fuel to be injected into the engine.
In order to determine, for example, the optimum amount of fuel to be injected into an internal combustion engine, air/fuel ratio control is performed to measure the amount of oxygen contained in the exhaust gas, whereby the air/fuel ratio in a combustion chamber can then be set to an optimal value.
In order to perform air/fuel ratio control, an oxygen concentration sensor is positioned in the exhaust manifold of the engine, so as to measure the amount of oxygen contained in the exhaust gas flowing therethrough, whereupon the fuel injection quantity is then corrected and re-calculated on the basis of a signal from the oxygen concentration sensor. The fuel to be injected into the engine is calculated on the basis of engine speed, the volume of intake air, and the like.
A conventional oxygen concentration detection apparatus comprises a sensor element which is constituted by a solid-state electrolyte material of a ZrO.sub.2 -based oxygen ionic conductor doped with Y.sub.2 O.sub.3, Yb.sub.2 O.sub.3, or the like. For example, a cup-like sensor element with a closed cylindrical bottom portion is constituted by a solid-state electrolyte material, on the inner and outer surfaces of which electrode layers are formed. The electrolyte material is exposed in an atmosphere of a gas being subjected to measurement, while at the same time, outer air as a reference gas is supplied to the interior thereof. The electrolyte material is then heated by a heater so as to accurately measure the oxygen concentration.
Since, in a conventional oxygen concentration determining apparatus, the cup-like sensor portion must be formed of solid-state electrolyte material, the fabrication process is therefore complicated. In addition, since it has a cup-like shape, it is difficult to increase the mechanical strength of the sensor portion. Therefore, when the cup-like sensor portion is attached to an internal combustion engine portion and subjected to vibration, many countermeasures must be taken. The structure of the heater is also complicated.
A sensor element as described in U.S. Pat. No. 4,282,080 has been proposed, with the aim of solving the above problems. The oxygen concentration sensor element described in this patent has a two-layered (a heater layer and an electrode layer) structure formed on each major surface of a solid-state electrolyte plate, to thereby constitute an integral structure. An outer air inlet opening is formed in a portion corresponding to one major surface of the sensor element of the solid-state electrolyte plate on which the heater and the electrode are integrally formed.
The laminated sensor element which surrounds the outer air inlet port is basically plate-like in shape. Therefore, the position of the element with respect to the direction of the gas flow inevitably adversely influences the accuracy of determining the oxygen concentration.
Since the element is plate-like in shape, it is difficult to obtain a sufficiently high mechanical strength. Since the electrode layers are formed on both surfaces of the solid-state electrolyte plate and the heaters are formed thereon, a plurality of insulating layers are required to electrically insulate the electrode layers from the heaters. As a result, the structure of the sensor element is complicated.
Another conventional oxygen concentration sensor is proposed in Japanese Patent Disclosure (Kokai) No. 61-272649. In this sensor, a plurality of grooves are formed on the outer surface of a rod-like core made of an electrically insulating material such as Ae.sub.2 O.sub.3. These grooves are used as channels for guiding outer air as a reference gas. An inner electrode, an outer electrode, and a heater are formed on each major surface of a solid-state electrolyte sheet. This solid-state electrolytic sheet is then wound around the outer surface of the core.
In the oxygen sensor having the structure described above, the grooves are formed to guide outer air as a reference gas. If the sensor is designed such that the volume of the core decreases so as to reduce, for example, the heat capacity, the groove cannot be as wide or deep as is desired. Since the width and/or depth of the groove is limited, it is difficult to smoothly guide outer air to a portion of the solid-state electrolyte layer, and to determine with accuracy the oxygen concentration in, for example, engine exhaust gas.